1. Field of the Invention
The invention relates to a filtering device used in wireless communication, especially to a multilayered band separator, which uses transmission lines as inductors and ground parasitic capacitors to shorten the required grounding transmission line length, thereby reaching a reduction size goal.
2. Description of Related Art
In wireless communication, multiband operation is widely employed. In a current wireless communication system design, a dual-frequency filter (duplexer) or a band separator in design generally considers the following factors: insertion loss, guard band, isolation, and outband rejection. When a separate band is very wide and has a strict demand on insertion loss and isolation, a lowpass-and-highpass filter is preferable. When the separate band is very wide and has a strict demand on insertion loss only, the combination of two bandstop filters is preferable. When size and outband rejection are important, the combination of two bandpass filters or a bandpass-and-bandstop filter is preferable. When the separate band is very narrow and has a strict demand on insertion loss, a bandstop-and-highpass filter is preferable. For example, for the GSM900/DCS1800 dual-band filter applied in Taiwan or the AMPS900/PCS1900 dual-band filter applied in USA, the separate band is wide and demands less insertion loss, so that a lowpass-and-highpass filter or the combination of two bandstop filters is preferable.
FIG. 1 is a typical multilayered band separator with the combination of two bandstop filters for a GSM/DCS dual-band handset. Two bandstop filters, Notch 1, Notch 2, respectively include a shunt capacitor and inductor. As shown in FIG. 1, when inputting signal at T1, the filter separately outputs the signal within 900 MHz-band at T3 and the signal within 1800 MHz-band or 1900 MHz-band at T2 due to the different LC response parameters. The output signals are shown in FIG. 2. Because the GSM/DCS dual-band handset meeting isolation bandwidth requirement at 20 dB is 10% or more of the entire bandwidth, the circuit mentioned above cannot meet isolation bandwidth requirement. Accordingly, referring to FIG. 3, an improved circuit is shown in U.S. Pat. No. 5,880,649 by Tai et al. In FIG. 3, besides two bandstop filters, a lowpass filter formed by a capacitor and a highpass filter formed by a shunt LC are included. As shown in FIG. 3, when signal T11 respectively passes through two bandstop filter Notch 3, Notch 4 and the corresponding lowpass and highpass filters, the output signal is separated respectively at f=900 MHz-band for T33 and at f2=1800 or 1900 MHz-band for T22. As shown in FIG. 4, the dual output signal response diagram illustrates such a circuit layout capable of enlarging the bandwidth at 20 dB to meet the system requirement by adding a stage filter. However, this makes the circuit more complicated and costs more.
Accordingly, an object of the invention is to provide a multilayered band separator with grounding parasitic capacitor, which uses a grounding transmission line as an inductor and a grounding parasitic capacitor to shorten the required grounding transmission line for the purpose of miniaturization and thereby having less insertion loss.
Another object of the invention is to provide a multilayered band separator with transmission line section, which converts an attenuation pole of a lowpass filter into an open-circuit pole and produces an attenuation pole on the low-frequency portion of a high-frequency port by a series lowpass filter, so that a better rejection effect is gained without an additional filter stage.
The invention provides a multilayered band separator with grounding parasitic capacitor, which uses a parasitic capacitor and a transmission line so as to have less insertion loss and better rejection. The multilayered band separator with grounding parasitic capacitor includes: a quasi-highpass filter, a semi-lumped lowpass filter and a phase shifter. The quasi-highpass filter includes a third capacitor connected to antenna port/input port, a fourth capacitor with an end connected to the first capacitor and another end connected to grounding, an inductive device with an end connected to grounding and another end connected to the connection point of the third and fourth capacitors, and a second capacitor with an end connected to a higher-band output end and another end connected to the connection point of the third and fourth capacitors and the inductive device to pass a high-frequency output signal. The phase shifter is a transmission line connected to the antenna port/input port. The semi-lumped lowpass filter includes an inductive device with an end connected to a lower-band output end and another end connected to another end of the phase shifter to produce a low-frequency output signal, and a first capacitor connected across the inductive device.
As noted above, the inductive devices are implemented in the form of transmission line, thus the invention presents the characteristics of the quasi-highpass filter, thereby having less insertion loss. The phase shifter is also implemented in the form of a transmission line, thus the invention can convert the attenuation pole of the semi-lumped lowpass filter into open-circuit pole and further produce an attenuation pole on the low-frequency side of quasi-highpass filter by applying the phase shifter connected in series with the lowpass filter, so that better rejection effect and additional isolation bandwidth are gained without an additional filter stage.